Discharge method and apparatus for generating plasmas

ABSTRACT

Two methods and corresponding electrode designs are provided for the generation of a plasma, for example, at or about one atmosphere. Using these methods, various webs, films and three-dimensional objects are beneficially treated in a reduced amount of time. A first method utilizes a repetitive, asymmetric voltage pulse to generate a plasma discharge between two electrodes. An asymmetric voltage pulse is used to generate a discharge in which a substrate can be exposed predominately to either positive or negative plasma species depending on the voltage polarity used. A second method uses the gap capacitance of an electrode pair and an external inductor in shunt to form a resonant LC circuit. The circuit is driven by a high power radio frequency source operating at 1 to 30 MHz to generate a uniform discharge between the electrode pair. Both methods have temperature controlled discharge surfaces with supply gas temperature, humidity and flow rate control. The gas flow is typically sufficient to cause a turbulent flow field in the discharge region where materials are treated. Electrode pairs implement these methods and include a metal faced electrode and a dielectric covered electrode, one or both of which have a series of holes extending through the electrode face for supply gas flow. The second of the above-described methods will also operate with paired, metal faced electrodes, but under more restricted operating conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 08/719,588, filed on Sep.25, 1996, now U.S. Pat. No. 5,895,558, which is a continuation ofapplication Ser. No. 08/492,193, filed on Jun. 19, 1995, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to methods and apparatus forgenerating a plasma at or about one atmosphere, especially for purposesof treating various webs and films to enhance their properties, and tothe treated webs and films, which have the improved and desirableproperties.

2. Description of the Related Art

The surface treatment of polymer materials using a plasma discharge canlead to a broad range of improved results. A plasma discharge can beused to initiate chemical reactions on the surface of a substrate orroughen a surface from ion bombardment. One important benefit that canbe achieved is to provide a more hydrophilic, or wettable surface.Plasmas produced under a vacuum have produced hydrophilic surfaces.However, this effect is typically short term for vacuum plasma treatedmaterials. Recent experiments using a one atmosphere dielectric barrierdischarge, with a sinusoidal excitation of a few kilohertz, haveproduced meltblown polypropylene samples which were wettable for eightmonths, and longer. However, the treatment times for these samples weregenerally on the order of four to five minutes, which is consideredrelatively long for practical applications.

By controlling certain processes of the plasma/substrate interaction,and by exploiting various features associated with a one atmospheredischarge, higher plasma power densities and shorter treatment times canbe obtained. When exposed to a plasma, a substrate will be bombarded byelectrons, ions, radicals, neutrals and ultraviolet (UV) radiation whichis sometimes sufficient to cause sputtering or etching of the exposedsurface. The resulting volatile products are likely to contaminate theworking gas and can be redeposited on the substrate. Sufficient gas flowwithin the discharge zone can minimize these problems. However, inaddition to etching and roughening the substrate, ions can reactchemically with the substrate.

The energy and flux of ions to the substrate can be significantlyincreased by biasing the substrate, usually to a negative potential.Controlled substrate biasing for a high pressure discharge requiresmetal faced electrodes or an asymmetric voltage waveform when using adielectric barrier discharge. A symmetric or sinusoidal waveform willalternately bias a substrate positively and then negatively throughout acycle, partially reversing the effects produced by each half cycle.

The energetic UV radiation produced by a plasma can have a variety ofeffects on both the background gas and the polymer substrate. Vacuum UV(primarily at short wavelengths, typically 50 to 250 nm) can causephotoionization and bond dissociation yielding free radicals. Radicalsproduced on a polymer surface can cause crosslinking of a polymer chainor react with species present in the gas phase. For the production of ahydrophilic surface, oxygen or oxygen containing radicals must typicallybe present. Since many competing reactions will occur in an oxygencontaining gas phase, and since these reactions will have temperaturedependent reaction rates, proper control of the background gastemperature will result in higher concentrations of the appropriatespecies to enhance a given surface treatment.

Ultraviolet production in a gas phase discharge can be enhanced by theuse of a gas with accessible emission lines (in the UV) for theoperating mode of the discharge. Proper electrode geometry with metalfaced electrodes reflective to UV, and a dielectric barrier transparentto UV, will enhance the UV levels in the gas discharge.

SUMMARY OF THE INVENTION

It is therefore the primary object of the present invention to providefor the improved treatment of webs and films, especially those formed ofpolymer materials, with a plasma generated, for example, at or about oneatmosphere of pressure and in a relatively short period of time.

It is also an object of the present invention to provide webs and films,especially those formed of polymer materials, which have been treatedwith a plasma generated, for example, at or about one atmosphere ofpressure to enhance their properties, especially in terms of theirwettability (hydrophilicity) or non-wettability (hydrophobicity).

It is also an object of the present invention to provide improvedmethods for treating such webs and films to enhance their properties,especially in terms of their wettability (hydrophilicity) ornon-wettability (hydrophobicity) as well as other desirable propertiessuch as printability, especially for films.

It is also an object of the present invention to provide methods fortreating such webs and films to achieve the foregoing improvements,which exhibit relatively short exposure times while avoiding thepotential for damage to the substrate which is to be treated.

It is also an object of the present invention to provide apparatus forimplementing the foregoing methods, for the treatment of webs and filmsto suitably enhance their properties.

It is also an object of the present invention to provide electrodedesigns for implementing the foregoing methods.

It is also an object of the present invention to provide correspondingcircuit designs for suitably exciting the electrodes of the presentinvention.

These and other objects which will become apparent are achieved inaccordance with the present invention by two methods and correspondingelectrode designs for the generation of a plasma, for example, at orabout one atmosphere.

A first method utilizes a repetitive, asymmetric voltage pulse togenerate a plasma discharge between two electrodes. An asymmetricvoltage pulse is used to generate a discharge in which a substrate canbe exposed predominately to either positive or negative plasma speciesdepending on the voltage polarity used. A second method uses the gapcapacitance of an electrode pair and an external inductor in shunt toform a resonant LC circuit. The circuit is driven by a high power radiofrequency source operating at 1 to 30 MHz to generate a uniformdischarge between the electrode pair.

Both methods have temperature controlled discharge surfaces with supplygas temperature, humidity and flow rate control. The gas flow istypically sufficient to cause a turbulent flow field in the dischargeregion where materials are treated. Such methods are generally intendedto operate within a metal enclosure to allow containment of the workinggas and to provide shielding of the electromagnetic fields.

The foregoing methods are preferably practiced with an electrode pairincluding a metal faced electrode and a dielectric covered electrode,one or both of which have a series of holes extending through theelectrode face for supply gas flow. The second of the above-describedmethods will also operate with paired, metal faced electrodes, but undermore restricted operating conditions.

A remarkable aspect of the present invention is that improved propertiescan be imparted to webs and films within a treatment period which isvery short. In accordance with the present invention, improvedproperties can be obtained by exposure to the plasma in sixty seconds,or less, frequently with treatments of less than 20 seconds, and quitesatisfactorily for periods of time as little as 1.5 seconds. Whensequential treatments are performed, the above-mentioned times refer tototal timed exposure to the plasma.

The invention can be practiced with a variety of gases, typically inertgases like helium and argon, active gases like oxygen and nitrogen, andmore complex gaseous molecules like carbon dioxide and ammonia. Gasesmay be used in mixtures (of two or more gases), including air, or asingle gas with oxygen or some other suitable gas. Gaseous mixturesincluding oxygen are preferably combined in relative proportionsincluding 2 to 20% oxygen. The gaseous mixtures may be essentially dry(i.e., essentially gaseous), or may be biphasic, such as a gascontaining relatively limited proportions of a liquid (e.g., watervapor). Additional gases which may be used for appropriate applicationswould include hydrogen (e.g., for saturating a polymer to create a morehydrophobic surface) and some of the fluorocarbons like CF₄.

For further discussion of the improved methods and electrodeconfigurations, and webs and films of this invention, reference is madeto the detailed description which is provided below, taken inconjunction with the following illustrations.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention willbecome more fully apparent from the following detailed description, theappended claims, and the accompanying drawings in which:

FIG. 1 is a schematic illustration of an apparatus for treating webs andfilms with a plasma generated at or about one atmosphere of pressure andin accordance with the present invention.

FIG. 2a is a cross-sectional view of an electrode useful in implementingthe apparatus of FIG. 1, having a dielectric face.

FIG. 2b is a top plan view of the electrode of FIG. 2a.

FIGS. 3a and 4a are cross-sectional views of other electrodes useful inimplementing the apparatus of FIG. 1, having an exposed metal face.

FIGS. 3b and 4b are top plan views of the electrodes of FIGS. 3a and 4a,respectively.

FIGS. 5a and 5c are cross-sectional views of alternative embodimentelectrodes that are capable of operating with an applied magnetic field.

FIGS. 5b and 5d are top plan views of the electrodes of FIGS. 5a and 5c,respectively.

FIG. 6 is a cross-sectional view illustrating operation of the electrodeof FIG. 2a and the electrode of FIG. 3a in combination.

FIG. 7 is a schematic diagram of a first circuit for exciting theelectrodes of FIG. 6.

FIGS. 8a and 8b are graphs showing typical voltage and current waveformsresulting from operations of the circuit of FIG. 7.

FIG. 9 is a schematic diagram of a second circuit for exciting theelectrodes of FIG. 6.

FIGS. 10a and 10b are graphs showing typical voltage, current andphotoemission waveforms resulting from operations of the circuit of FIG.9.

FIGS. 11a and 11b shows two views of an electrode configuration usefulin producing a plasma for the treatment of three-dimensional objects,the upper view of which is a cross-sectional view of the electrodeconfiguration and the bottom view of which is a top plan view of theelectrode configuration.

DETAILED DESCRIPTION

FIG. 1 generally illustrates an apparatus 1 for treating webs and filmsin accordance with the methods of the present invention. The apparatus 1may be used to treat any of a variety of webs and films, primarily forpurposes of enhancing their hydrophilic properties and theirwettability, and also their hydrophobic or non-wetting properties, andin particular for films, to enhance printability. In addition to themore conventional polypropylene, polybutadiene webs and films, webs andfilms formed of polypropylene--polyethelene copolymers, poyesters,styrene copolymers, styrene butadiene, nylon 12, and others, may betreated. This can include both spunbond and meltblown webs, nonwoverwebs and films. The webs and films may be non-porous or porous. In thediscussion which follows, such materials will generically be referred toas a "substrate" 2. The term "gas" will generally refer to a single gasor a mixture of two or more gases.

The treatment apparatus 1 is generally made up of an electrodeconfiguration 3, a system enclosure 4 and a gas handling system 5, whichare used to generate a plasma at or about one atmosphere of pressure andto expose the substrate 2 (preferably a polymer substrate) to thegenerated plasma. The electrode configuration 3 is made up of paireddischarge electrodes 6, 7 housed within a metal enclosure and supportedin position using high dielectric support rods 8. The gas handlingsystem 5 operates to supply the electrodes 6, 7 with a temperature,humidity and flow rate regulated working gas, which in turn flowsthrough the opposing faces 9, 10 of the electrodes 6, 7, as will bediscussed more fully below. The lower electrode 7 is configured so thatit can be pressurized either positively or negatively with respect tothe enclosure 4. This is done to establish a flow of gas from the face10 of the electrode 7, for the treatment of films, or a flow of gas intothe electrode 7, for the treatment of porous materials.

Each of the electrodes 6, 7 receives a gas flow through a communicatingmanifold 11, 12. Preferably, the working gas is at least partiallyrecycled. To this end, an oil-free compressor 13 is provided forpurposes of establishing the necessary flow, and the working gas isfiltered (at 14) and then chilled (at 15) to remove any moisture. Aheating element 16 is provided for reheating of the working gas, whichmay be required depending on the operating conditions. Some of theworking gas may be vented (at 17) and/or replaced with bottled supplygas (at 20), as needed, to establish an appropriately controlled flowrate. The supply and vent flow rates are usually adjusted so that thesystem enclosure 4 is at a slight positive pressure with respect toatmospheric pressure. Suitable flow meters 18 are provided atappropriate locations for monitoring this process.

Typical bottled supply gas flow rates are from 5 to 40 liters perminute. The gas recirculation rates will vary from 10 to 300 liters perminute. Typical gas and electrode temperatures are from 25 to 70° C.,for the pulse discharge method to be described below, while colder gasand electrode temperatures are required for the resonant LC method to bedescribed below (due to gas heating in the sheath regions). In general,the highest possible working temperature which does not allow thermaldamage to the substrate, will produce the best results for hydrophilicsurfaces.

Both of the electrodes 6, 7 are further supplied with a temperaturecontrolled working fluid. For example, a glycol-water solution can beused for liquid cooling. The working gas supplied to the system can beused for gas phase electrode cooling. The desired working fluid flowsthrough a manifold 19, 20 which communicates with an internal coil ineach electrode, as will be discussed more fully below. This providesadditional control of the temperature and discharge volume of theelectrodes 6, 7. Temperature control is provided responsive to a heatexchanger 21 in communication with the manifolds 19, 20. Dischargevolume control is provided responsive to working gas pressures developedby the gas handling system 5, through a communicating conduit 22.

The substrate 2 to be treated is conveyed through the discharge volumedefined by the enclosure 4, and between the electrodes 6, 7, at acontrolled rate. To this end, a supply reel 23 and a take-up reel 24 areused to treat a continuous length of material. In FIG. 1, the supplyreel 23 and the take-up reel 24 are shown within the enclosure 4. It isequally possible to position a supply reel and a take-up reel outside ofthe enclosure 4. However, in such case, a suitably sealed entrance andexit must be provided to allow the substrate 2 to pass through theenclosure 4, and between the electrodes 6, 7.

The substrate 2 is also conveyed past a set of spray nozzles 25, whichcan be used to post-treat the activated surface of the substrate ifdesired. Such post treatment can include, for example, the applicationof a polar-group chemical (such as an alcohol or acetone) directlyfollowing plasma treatment to "lock" or "fix" the treated surface forimproved wettability. Other post-treatments are equally useful ifindicated for a particular application. In any event, a supply cylinder26 and a supply pump 27 are provided for delivering the post-treatmentmedium to the spray nozzles 25.

FIGS. 2a and 2b illustrate an electrode 30 (either the electrode 6 orthe electrode 7 of FIG. 1) which is covered with a dielectric to reducethe potential for arcing. The electrode body 31 is preferably machinedfrom solid stock (a metal conductor), usually formed of copper or astainless steel The metal face 32 is machined, as shown, eventuallyproducing a curved surface which is approximately hyperbolic in shape.The precise shape of the electrode face 32 will necessarily bedetermined empirically, through an interactive process involving testingof the electrode without a dielectric cover and observing wherebreakdown is initiated. A uniform (flat) radius can also be used, ifsufficiently large, but will generally result in a poor usage ofelectrode body size. Uniform radii of at least 2 cm are required tominimize arcing on the electrode's edge.

A dielectric layer 33 is cemented (at 34) to the face 32 of theelectrode 30, preferably using a high grade epoxy or a ceramic adhesive.Dielectric materials such as Pyrex™ glass, Vycor™ glass, Macor™ (amachinable ceramic) and Amalox 68™ (a fired alumina ceramic), have beenused with satisfactory results. Dielectric thicknesses of from 3 mm to 7mm have been used. The thickness used is governed by the material'sdielectric constant, the loss factor which determines internal healingdue to the electric field, the mechanical properties of thermal shockresistance, thermal conductivity, flexural strength and machinability. Alow vapor pressure epoxy (Torr-Seal™) was used for mounting thedielectric layer 33 to the electrode face 32.

A radial pattern of holes 35 is machined through the face 32 of theelectrode 30. Each of the holes 35 is preferably fitted with adielectric sleeve 36, counter sunk into the back 37 of the dielectricface 32. A machinable ceramic material (such as Macor™) or alumina tubesare preferably used to form the sleeves 36. The sleeves 36 arepreferably long enough so that they extend beyond the inside surface 37of the electrode 30 by at least 3 mm. This is required to prevent arcingto the back side 37 of the electrode face 32. Small flow holes,typically number 60 to number 55, are machined through the dielectricsleeves 36 and the dielectric layer 33. The interior of the electrode 30is further machined to form a cavity 38 which acts as a plenum so thatgas flow is more evenly distributed to the outlet holes 35.

A coil network 39 is inserted into the cavity 38 and soldered to thewall 40 of the electrode 30. The coil network 39 communicates with thepreviously described manifolds 19, 20 (and the heat exchanger 21) toallow a temperature controlled fluid to be circulated through the coils39 to regulate the temperature of both the electrode 30 and the workinggas received in the cavity 38. A cover plate 41 is fitted to the openend 42 of the electrode 30 and acts as a gas barrier, as well as amounting plate for the electrode 30. A gas inlet port 43 is provided inthe cover plate 41 to establish communication between the cavity 38 andthe manifolds 11, 12 which supply the electrodes with the working gas.

FIGS. 3a and 3b illustrate a metal faced electrode 45, which is usedopposite the dielectric covered electrode 30 to develop the electrodes6, 7 which make up the electrode configuration 3 of FIG. 1. The metalfaced electrode 45 is machined similar to the dielectric coveredelectrode 30, except for the holes 35 and the associated dielectricsleeves 36. In this case, the radial hole pattern for the metal facedelectrode 45 will generally have significantly more holes 35' than thedielectric covered electrode 30. The holes 35' are directly sized at anumber 60 to a number 50, as opposed to the 3 to 5 mm undercuts requiredfor receiving the dielectric sleeves 36.

Since any hole in the electrode face will locally distort the electricfield, it is preferred that the total hole area (in sum) not exceed 25%of the electrode face area for the dielectric covered electrodes 30. Dueto electric field distortion and the need to produce good gas mixing,the holes 35, 35' in the two electrode faces 32, 32' should be offsetazimuthally and/or radially. The number of holes 35, 35' in theelectrode faces 32, 32' will vary widely, depending on variousparameters and conditions. As examples, as few as 7 holes (number 60)have been used for a 4 inch diameter copper electrode, while as many as108 holes have been used for a 3.5 inch diameter brass electrode. Moreor fewer holes may be used for other applications.

FIG. 4a further illustrates an alternative temperature regulatingarrangement for the electrodes of the present invention, which isequally applicable to the dielectric covered electrodes 30 and lo themetal faced electrodes 45. In this case, the wall 40' of the electrode45 is machined to develop a stepped recess 46. The recess 46 is in turnjacketed with a sleeve 47. Inlet and outlet connections 48, 48',respectively, are provided to establish an appropriate fluid flow. Suchan arrangement is more suitable for electrode bodies formed fromaluminum and stainless steel. Use of the coils 39 for purposes oftemperature regulation is more suitable for electrode bodies formed fromcopper or brass.

The working gas introduced into the electrodes 30, 45 is typically at250 to 500 torr above atmospheric pressure, with a flow rate ofapproximately 10 to 200 liters per minute for a 10 cm (diameter)electrode. The flow rate will vary depending on the type of gas used andthe discharge technique employed. The pulse discharge technique whichwill be discussed more fully below will typically use high flow rates todelay and disrupt the formation of a filamentary discharge. Such flowrates produce a Reynolds number in the range of from 1,000 to 100,000(in a flow hole). Hence, the flow is typically very turbulent at thehole opening and into the discharge region. This turbulent flow allowsimproved temperature control of the gas phase, and of the substrate, aswell as a rapid removal of etched products. When used to treat porousmaterials, the lower electrode (the electrode 7 of FIG. 1) is used as agas return to draw flow through the substrate material. For thetreatment of polymer films, a positive flow through both electrodes isused to keep the film suspended between the electrodes, eliminating theproblem of film-to-electrode adhesion.

FIGS. 4a and 4b illustrate an electrode 45', the structure of whichsubstantially corresponds to the structure of the electrode 45 of FIGS.3a and 3b except for the addition of a partition wall 49. The partitionwall 49 is placed within the electrode cavity 38' to partition thecavity and allow two separate gas mixtures to be used simultaneously. Asan example, and for reasons which will be discussed more fully below, afirst gas can be introduced into the cavity partition 38", for exposingthe substrate 2 to a first active species, while a second gas isintroduced into the cavity partition 38'", for exposing the substrate 2to a second active species. In this way, the substrate 2 can besubjected to varying treatments. A similar result can also be obtainedwith two separate pairs of electrodes (the electrodes 30 or theelectrodes 45), supplied with different gases for exposing the substrate2 to different active species as the substrate 2 is conveyed through theresulting electrode configuration.

The electrodes 30, 45 are also capable of operating with an appliedmagnetic field. FIGS. 5a through 5d illustrate two implementations 50,50' of the previously described electrode designs with a magnetic field.In FIGS. 5a and 5b, the applied magnetic field is developed by permanentmagnets 51 mounted inside (one or both of) the electrodes. The developedfield is essentially perpendicular to the face 32' of the electrode 50,and serves multiple functions. For example, the developed field tends toconfine charged particles to the plasma discharge region. The developedfield further interacts with the radial electric field produced by theshaped outlets of the gas flow holes 35' (in the metal faced electrode).The radial electric field produced locally by a flow hole, coupled withthe axial magnetic field, will produce an E×B azimuthal force on chargedparticles, resulting in particle heating.

The presence of a magnetic field also acts to improve the treatment ofweb materials. To this end, thermalized ions will tend to spiral alongmagnetic field lines, with azimuthal velocity components and a linearvelocity component along the magnetic field lines. The azimuthalvelocity components due to the imposed magnetic field will improve theexposure of individual web fibers. By using a magnetic field intensityadequate to produce an ion gyro-radius comparable to the fiber spacingsin a web, a greater exposure of the fiber surface to active ion speciescan be achieved. Magnetic fields of a few hundred gauss are typicallyrequired for this.

In the foregoing discussion the permanent magnets are oriented with thesame polarity positioned toward the electrode face 32' (either north orsouth). The magnets can also be used with alternating polarity. Thisproduces some regions of magnetic field perpendicular to the dominantelectric field, which is perpendicular to the electrode face. Mountingof the magnets is facilitated with this arrangement, since field lineson the backside of the magnets can be connected using a suitableferromagnetic metal (i.e., soft iron). This will tend to better hold themagnets in position.

In FIGS. 5c and 5d, the applied magnetic field is developed by anelectromagnet 52 mounted external to (one or both of) the electrodes.The electromagnet 52 surrounds the electrode 50', and is electricallyisolated from the electrode. The electrical connection with theelectromagnet 52 is routed so that the connection does not complete acurrent loop with the magnet coil. The large inductance of the magnetcoil would ordinarily tend to degrade the high frequency response of theelectrical circuit used to operate the electrode 50'. The magnet coil isdesigned so that it can be energized with either a direct current or alow frequency sinusoidal current (typically 60 Hz). The use of amodulated magnetic field produces a range of ion gyro-radii for ionspenetrating into a web substrate.

FIG. 6 schematically illustrates two opposing electrodes (e.g., theelectrodes 6, 7 of FIG. 1) and a polymer substrate 2 which is beingplasma treated. The discharge gap 55 (typical spacings of from 0.6 mm to10 mm have been used) is enlarged for purposes of illustration. In thiscase, the gas flow is configured for treating a film material, aspreviously described, and keeps the film suspended between theelectrodes 6, 7 to prevent adhesion to either of the electrode faces 32,32'. As shown, the upper electrode 45 is biased negatively relative tothe lower electrode 30, to drive negative species of the plasma into thetop surface 53 of the substrate 2. These species will interact with thesubstrate 2 and produce volatile products which can be redeposited andcontaminate the working gas in the discharge volume (at 56). Gas flowthrough the faces 32, 32' of the electrodes 30, 45 reduces theseeffects.

Ion bombardment and ultraviolet (UV) irradiation of the metal facedelectrode 45 will produce secondary electrons and photo-emittedelectrons. These electrons are important in sustaining a high pressuredischarge. Electronegative gases such as oxygen and carbon dioxide havevery high attachment rates for electrons and tend to extinguish theplasma.

Ultraviolet irradiation and ion bombardment of a polymer substrate willresult in bond dissociation and substrate etching. The polymer chain cancross link or react with the active species present in the dischargezone to produce a modified surface. The processes of etching and bonddissociation are necessary for the surface modification of a substrate,but they are also competing processes for the production of a modifiedsubstrate with new species attached to the polymer chain. UV and ions ofsufficient energy to cause bond dissociation and etching of the polymerchain will also cause bond dissociation and etching of the modifiedpolymer chain. Hence, some equilibrium is reached for a given gasmixture and discharge conditions, and a modified polymer substrate isproduced.

Control of the discharge current-voltage characteristics, gas flow, gastemperature and gas mixture can improve the production of the modifiedpolymer substrate. However, a given plasma will always tend to havecompeting processes for polymer surface modification. A two step processwhich can be used to partially solve this problem is to first expose thesubstrate to a discharge plasma having strong UV and/or etchingproperties, activating the substrate surface, and to then expose thesubstrate to a plasma with high concentrations of active speciessuitable for reactions with the activated polymer surface. Partitionedelectrodes 45' such as are illustrated in FIGS. 4a and 4b, using twodifferent gas mixtures appropriate to each step of this two stepprocess, are useful in implementing such a treatment. As previouslyindicated, two separate paired electrode configurations are also usefulin implementing such a treatment, if desired.

One method which is useful in exciting the previously describedelectrodes (30, 45) is to employ a pulse discharge circuit including apulse generator, pulse transformer and high voltage diodes to generate aone atmosphere plasma between the two shaped electrodes. FIG. 7illustrates one such arrangement of these components, to form a pulsedischarge network 60. A high voltage pulse generator 61 (e.g., a VelonexModel 570 or equivalent) is used to develop a pulse having an amplitudeof at least 1 kv, a variable pulse width of 1 to 20 microseconds, and avariable pulse repetition frequency of 1 to 100 kHz. Peak pulse power ison the order of 20 kw, with an average power of 200 watts or more. Toincrease the output voltage of the pulse generator 61, and to bettermatch the generator impedance R_(g) (R_(g) is 200 ohms for the VelonexModel 570) to the plasma load Z_(p), a step-up voltage transformer 62 isused. Transformer turns ratios of 1:2 to 1:7 are typically used,depending on the working gas and the electrode geometry and spacing. Theuse of electronegative gases such as oxygen and carbon dioxide requireshigher voltages, and hence, a higher turns ratio than the use ofstrictly inert gases such as helium or argon. When the pulse source ischarge limited for a given electrode size and voltage, the use of asmaller diameter electrode will allow higher voltages to be obtained(and the use of a higher turns ratio transformer).

Since the discharge electrodes represent a high capacitance load (thesum of 2×C_(sh) +C_(di)), the diodes D1, D2 and the correspondingresistors R1, R2 are additionally used to control the voltage waveformand current flow. The diode D1 and the series resistor R1 act as avoltage clipping network to reduce the amplitude of the "kick-back"voltage produced during the fall of the generator pulse. This voltage isdue to energy storage in the mutual inductance (coupling) between theprimary and secondary windings of the step-up transformer 62.

The positive (or negative) pulse from the generator 61 also causescharge to be displaced across the discharge electrodes, resulting in alarge electric field across the discharge gap (i.e., the gap 55 of FIG.6) and breakdown of the working gas. When the pulse is terminated or canno longer be sustained, this displaced charge will tend towardequilibrium, and will appear as a sudden reverse current and voltagespike. The second diode D2, and the resistor R2 in shunt with the diodeD2, act to slow this reverse current and decrease the resulting voltagespike.

The combination of the diodes D1, D2 and the resistors R1, R2 helps toproduce a discharge that is predominately positively biased (ornegatively, depending on the lead connections at 63), for a polymersubstrate 2. Paired comparison tests of this method on a polymer filmtreated in a carbon dioxide plasma have verified the utility of usingthis asymmetric voltage pulse to excite a discharge. For example, apolyethylene-polypropylene copolymer treated with a carbon dioxideoxygen plasma had better wettability with a negatively biased upperelectrode than with a positively biased upper electrode.

The power delivered to the plasma discharge load Z_(p) must be coupledacross the combined capacitances C_(di) and C_(sh), where C_(di) is dueto the dielectric barrier and C_(sh) due to the plasma sheaths that formon the face of each electrode. These capacitances tends to limit thecurrent that can be delivered to the load Z_(p) for a given voltage.Increasing C_(di), e.g., by using a dielectric barrier with a highdielectric constant, will increase the discharge current (and power) tothe load Z_(p). The sheath capacitance C_(sh) can be partiallycontrolled with the gas flow established at the electrode faces. Theimpedance Z_(p) is often modeled as a three component network includingan inductance L_(p) in parallel with a capacitance C_(p) and aresistance R_(p). At high pressures, the plasma capacitance C_(p)becomes large, making the discharge predominately capacitive.

FIGS. 8a and 8b illustrate current and voltage waveforms for a singlepulse of a 10 kHz pulse repetition rate signal (the illustratedwaveforms are inverted since a negative pulse was used). Such a signalcan produce a plasma discharge in carbon dioxide, oxygen and helium gasmixtures. Plasma power densities on the order of 10.6 watts/cm³ areobtained. The resulting plasmas have been used to treat spunbondpolypropylene samples for treatment times of 15, 8 and 5 seconds,respectively. Each of the samples were made wettable to water as aresult.

In each case illustrated (FIGS. 8a and 8b), the voltage rise time was1.25 microseconds, during which current flowed to the electrodes. Theactual discharge was ignited at about 5 kv, where a small spike occursin the current waveforn. At the pulse termination, a fall time of about250 nanoseconds was observed, and a 6 kv inverted voltage pulse of 500nanoseconds was observed. This pulse would be approximately twice aslarge without the diode D1 and resistor R1 of FIG. 7. A current pulsealso occurs during this period. Since charge is conserved, the area ofthis inverted pulse waveform will be equal to the area of the initialcurrent pulse during the rise in voltage.

A pulse generator 61 having a fast rise time, with a properly designedpulse transformer 62 (matching R_(g) to Z_(p)), will also increase thedischarge current. Using the Velonex Model 570 pulse generator mentionedearlier, and a 1:5 toroidal wound pulse transformer, a 15 kv pulse hasbeen produced with a rise time of less than 1 microsecond. Plasma powerdensities of 6 to 20 watts/cm³ were produced. These discharge powerdensities have been used to treat polymer films and polypropylenespunbond fabrics with treatment times as short as 5 seconds.

The stray capacitance C_(st) of FIG. 7 is due to support rods,connecting tubes, and the proximity of high voltage surfaces to groundedconductors, and should be minimized. This capacitance requires increasedsupply current in order to produce a given voltage across theelectrodes. For this reason, the electrodes are preferably supported byhigh dielectric insulating rods and are temperature controlled using agas phase cycle, or a liquid cycle with a sufficiently long coolantpath.

Another method which is useful in exciting the electrodes (30, 45)previously described is to employ a tuned or resonant circuit, developedby connecting an inductor in parallel with the pair of dischargeelectrodes. The shunt inductor will be placed in parallel with thesheath capacitance formed on the face of each electrode and the plasmacapacitance formed by the discharge. At and near resonance, this circuitwill have a high impedance which is predominately real. This allows ahigh voltage to be generated across the circuit, and the breakdown of aworking gas between the discharge electrodes. A large recirculatingcurrent will flow through the plasma and shunt inductor. This currentwill increase the power that can be dissipated by the discharge plasmaand provide stability. The recirculating current will also enhance theelectron population generated by secondary emissions from the faces ofmetal electrodes. These secondary electrons, as well as electrons due toUV photoemission, play an important role in sustaining a high pressureradio frequency (RF) discharge operating in gases such as oxygen andcarbon dioxide, which have high electron attachment rates.

FIG. 9 illustrates one such arrangement, showing a preferred network 65(and its components) for exciting a uniform plasma between two of thepreviously described shaped electrodes. The bulk plasma is modeled as aninductance L_(p) in parallel with a capacitance C_(p) and a resistanceR_(p). Under suitable vacuum conditions, the plasma can be operated asself-resonant due to the parallel combination of L_(p) and C_(p). A highpower, radio frequency (RF) source 66 is used to provide at least 1 kwof RF power at 1 to 30 MHz. Tested systems have been operated at 13.56MHz and 2.2 kw. However, the network will, in practice, operate over theentire band of 1 to 30 MHz. The unbalanced (50 ohm) output of thegenerator 66 is converted to a balanced output voltage using animpedance transformer 67 having an impedance ratio of 1:1 to 1:9.

The transformer 67 (the voltage) is coupled to a symmetric "pi" matchingnetwork 68 with variable capacitors C1 and C2, and variable inductors L1and L2. For this arrangement, the capacitors C1 and C2 are variable,preferably over a range of from 20 to 450 pf for C1 and from 10 to 200pf for C2. The inductors L1 and L2 have the same number of turns, andare preferably variable over a range of from 2.5 to 5 μH. The network 68is tuned to match the output impedance of the transformer 67 to theimpedance of the resonant circuit formed by the shunt inductance L_(s)and the sheath capacitances C_(sh), C_(sh) in series with the plasmacapacitance C_(p). The series resistance R_(p) is the dischargeimpedance due to plasma ionization and heating, UV emission, particleloss, and neutral gas heating. The capacitance C_(p) formed when aplasma is present is not the same as the free space gap capacitanceformed by the two electrodes. Hence, a retuning process is required asthe plasma is initiated. Retuning is first initiated by adjusting thevariable inductor L_(s) to reduce the power reflected to the generatorsource. The pi matching network is then tuned to improve the matchbetween the generator and plasma load. Repeating this procedure canproduce a match with 10% or less reflected power.

A balanced pi network is used so that a push-pull current is drivenacross the discharge electrodes. An equivalent "tee" matching networkcan also be used to achieve an equivalent result. However, a pi networkis preferably used because it is somewhat easier to assembleexperimentally. In practice, the inductors L1 and L2 must be carefullytuned so that the voltages +V_(d) and -V_(d) are 180 degrees out ofphase. At and near resonance, large currents will flow through theinductor L_(s) and the discharge. This recirculating current istypically 3 to 10 times the supply current. Hence, the "Q" of theresonant circuit is typically 3 to 10. The discharge and pi networkshould therefore be adequately shielded and good high voltage RFtechniques observed. The use of a balanced pi matching network allowssome independent control of the voltage-current relation of thedischarge. This relation is normally fixed by the impedance of the load,or plasma parameters in this case. Since C2 is in parallel with L_(g)and the discharge, tuning and detuning of the circuit's resonantfrequency is direct, and allows the forward power delivered to theplasma to be varied.

For use with the metal faced electrodes 45, the network 65 is preferablyequipped with a DC power supply PS1, for electrode biasing. Blockinginductors Lb1 and Lb2 are installed to isolate the DC power supply PS1from high voltage RF. Typically, the inductors Lb1 and Lb2 are 50 μH, otlarger. Two blocking capacitors, Cb1 and Cb2, must also be used(typically 1000 pf) to prevent the DC power supply PS1 from shortingthrough the inductor L_(s) and the RF transformer 67.

This discharge technique will not only operate with the pairedelectrodes 30, 45 discussed earlier, but also with two metal facedelectrodes 45. The use of two metal faced electrodes 45 is desirable forseveral reasons. First, this allows a closer electrode spacing forhigher electric fields. Second, this allows a DC voltage bias to beapplied, increasing the flux of a given ion species to the substrate.Third, the metal faced electrodes 45 provide high reflectance surfacesto any UV radiation generated in the plasma. A metal surface with a highphotoelectric emission such as copper or gold will also provideadditional electrons. These electrons will assist in maintaining aplasma in electronegative gases.

FIGS. 10a and 10b illustrate voltage and current waveforms typical for aresonant discharge using helium, oxygen and nitrogen gases, with adielectric covered electrode 30. The measured voltage is one-half thedischarge voltage since it is measured with respect to ground. Thegeneration of a second harmonic is apparent at the peak of the voltagewaveform. The measured current is the supply current (for the circuit ofFIG. 9). The forward power delivered to the pi network was 1200 wattsand the reflected power was 400 watts, yielding a discharge power ofapproximately 800 watts.

FIG. 10c illustrates the voltage waveform for a resonant discharge (thedischarge of FIG. 10a for the circuit of FIG. 9), and the voltage outputof a photomultiplier tube viewing the plasma. This shows the plasmalight output to be uniformly modulated at twice the generator frequency.Since this signal is a continuous waveform, absent of flat or zerovoltage regions, the discharge is sustained continuously. The lowfrequency (1-10 kHz) dielectric barrier discharge will actually turn offmany times during a voltage cycle. Operating at high frequencies (1 to30 MHz), the resonant discharge has energy continuously supplied to theplasma at a rate fast enough to prevent plasma extinction, approaching atrue glow discharge.

Because of the high frequency and resonant circuit design, much higherpower densities are possible than with low frequency dielectric barrierdischarge methods. Using the 13.56 MHz source mentioned earlier, a 1.2kw discharge has been excited in a helium-oxygen plasma to produce aplasma with a power density of 50 watts/cm³. This power density is overone hundred times higher than power densities measured elsewhere. Due tothe level of gas heating which occurs in the plasma sheaths of aresonant LC discharge, the treatment of spunbond webs and films ispreferred to the treatment of relatively thick meltblown materials.Spunbond materials and films tend to be better positioned within thedischarge gap.

The electrode configurations illustrated in FIG. 1 and FIG. 6 are suitedprimarily for the continuous treatement of a nonconducting pliablematerial, preferably in the form of a web, film, sheet, yam or filament.However, since the treated material can occupy as little as 10% of thedischarge volume (for a film) to as much as 85% of the discharge volume(for a meltblown web), a considerable range of material thickness andtypes can be treated in accordance with the present invention. Web typesof spunbond, meltblown, hydroentangled, carded, needle punched andcomposite, layered or laminated materials can be treated, and theirsurface characteristics improved. Smooth or textured films can also betreated.

The discharge techniques mentioned above, coupled with electrodes havingcontrolled temperature and gas flow, allow a variety of different gasesto be used, and hence, a broad range of synthetic and natural polymermaterials to be treated. The vinyl polymers, polyethylene, polypropyleneand polystyrene can be treated. Webs or films of polyester, polyethyleneterephthalate (PET), and polybutylene terephthalate (PBT), as well asnylons, silicones and polycarbonates (Lexan), are well suited fortreatement. Natural materials such as cotton, wool, leather and papercan also be treated in accordance with the present invention, either assuch or as components of laminates, composites or of other materials tobe treated.

The foregoing electrode configurations 3 (comprised of the electrodes30, 45), shown in FIG. 1 and FIG. 6, are suited primarily for thetreatment of thin webs and films due to the small discharge volume whichis created. The plasma treatment of a three-dimensional object, such asa bottle, requires the production of a plasma that will exist outside ofthe interelectrode discharge gap (the gap 55 of FIG. 6). This can beaccomplished using a dielectric covered electrode in combination with agrid electrode. FIG. 11 illustrates one such arrangement 70, combining adielectric covered (shaped) electrode 30 and a plate electrode 71 withmultiple holes 72. The plate (or grid) electrode 71 is supportedparallel to the dielectric covered electrode 30 using an appropriatedielectric support 73. The support 73 also acts as a gas barrier for thesupply gas which is fed into the resulting interelectrode dischargevolume 74. The supply gas is introduced through four ports 75 which arearranged so that the longitudinal axis of each port 75 is tangential tothe edge 76 of the grid. The plate electrode 71 includes a grid pattern77 defined as an arrangement of uniformly spaced holes 72 located withinan area corresponding to the flat portions (the face 32) of the lowerelectrode 30. Since cooling of the plate electrode 71 is limited toconvection from the supply gas and radiative cooling, this electrodeshould be constructed of a heat resistant metal (e.g., a 310 or 309stainless steel). The openings of the grid pattern should collectivelyrange from 20 percent to 60 percent of the total surface of the plateelectrode 71.

The dielectric barrier layer 33 used for this configuration is typicallythicker than that used with a pair of shaped electrodes as previouslydescribed (i.e., as in FIGS. 2a and 2b). The edges of the grid patternholes 72 tend to produce a less uniform plasma, and more thermal stresson the dielectric. For this reason, the cavity 78 of the shapedelectrode is preferably filled with a temperature regulated fluid. Thepermanent magnets 51 discussed earlier can also be positioned within thecavity 78, if desired.

Once a plasma discharge is initiated with the arrangement 70, a plasma(and active species) will escape the discharge zone 74 with the supplygas, developing plumes 79. Ultraviolet radiation will also pass throughthe grid holes 72. A substrate (or other object) to be plasma treatedcan then be manipulated in the plasma plumes 79 generated by thisarrangement. Such a plasma discharge has been sustained in carbondioxide, with resonant LC excitation using the network 65 of FIG. 9. Theplate electrode 71 used had a 40% grid opening, with holes 72 having adiameter of 3.2 mm. The discharge power (of the power source 66) wasclose to 1 kw for a 9 cm grid diameter. No magnets were used in theshaped electrode 30, which had a diameter of 10 cm, and which included aPyrex™ dielectric having a thickness of 3.2 mm.

The foregoing can also be used for the treatment of conductingmaterials. Since the conducting materials will in such case be treatedexternal to the discharge zone, they can be kept electrically isolatedfrom the excitation network.

The pulse discharge technique and the resonant LC discharge techniquehave been used with the electrode configuration shown in FIG. 6 to treatboth polymer films and spunbond web materials. Polypropylene andpolypropylene--polyethylene copolymer blends have been treated mostfrequently, due to more immediate commercial interests. The followingtable (Table 1) lists treatment conditions and results for fourdifferent samples. Both treatment techniques used a 10 cm (diameter)lower electrode having a 3 cm thick Macor™ dielectric cover. Samples 1and 2 were treated with the pulse discharge technique, and employed a7.6 cm (diameter) brass upper electrode. Samples 3 and 4 were treatedwith the resonant LC discharge technique, and used a 10 cm (diameter)copper upper electrode.

                                      TABLE 1                                     __________________________________________________________________________                      Electrode                                                                           Electrode, Gas                                                                       Power                                            Sample Treated Material Gas Mixture, Flow Rate, Temperature Density                                                Number Treatment Time Blend                                                  .sup.2 LPM.sup.1 ° C.                                                  Watts/cc Results                        __________________________________________________________________________    Pulse Discharge Technique                                                     1   Polypropylene -                                                                       CO.sub.2, O.sub.2                                                                   150   60     12.7   Reduced contact                            polyethylene 7.6:4.3    angle from 83° to                              copolymer film, 5     30°                                              second exposure                                                              2 Polypropylene He, O.sub.2 180 38 7.5 Sample wettable,                        spunbond 15 9.5:7.5    absorbtion time ≦                               gr/m.sup.2, 5 second     3 sec. (3 month                                      exposure     plus lifetime)                                                Resonant Discharge Technique                                                  3   Straight                                                                              He, O.sub.2                                                                         28    18     18.4   Reduced contact                            polypropylene 10:1.5    angle from 80° to                              film (7 mil), 1.6     40°                                              second exposure                                                              4 Polypropylene He, O.sub.2 35 20 26 Sample wettable,                          spunbond 30 5:1    absorbtion time ≦                                   gr/m.sup.2, 1.6 second     10 sec.                                            exposure                                                                   __________________________________________________________________________     Notes:                                                                        1. LPM is liters per minute.                                                  2. Blend ratio is the volume flow rate ratio of the bottled supply gases.

The methods and electrodes of the present invention may be applied to agreat variety of substrates. Such substrates can include, for example,bonded carded webs, spunbond webs or meltblown webs. The meltblown websmay include meltblown microfibers. The substrates treated in accordancewith the present invention may have multiple layers such as, forexample, multiple spunbond layers and/or multiple meltblown layers.

The substrate treated in accordance with the present invention may bethermoplastic resins, which include polyolefins such as polyethylene,polypropylene (including high density polyethylene), ethylene copolymers(including EVA and EMA copolymers with high tensile moduli), nylon,polyamids, polyterathalates, polyesters, polystyrene,poly-4-methylpentene-1, polymethylenemethacrylate, halogenatedpolyoefins such as fluoro- or chloro-substituted polyolefins such aspolytrifluorochloroethylene, polyurethanes, polycarbonates, silicons,polyphenylene sulfide, and others. Other polyolefins, polyesters andpolyamids are described in U.S. Pat. No. 5,965,122, which isincorporated herein by reference.

The polymers may be elastomeric or non-elastomeric. They may behydrophilic or hydrophobic, or indifferent in that respect. The filmstreated in accordance with the present invention may be elastomeric ornon-elastomeric. They may be porous or non-porous (impervious to gasesand/or liquids). It is noteworthy that in accordance with the presentinvention, various characteristics of the surface of a film may bealtered, specifically, to impart desirable properties.

Printability with various dyes and prints may be improved. Polyolefinfilms and other films of polymeric materials are notoriously difficultto print. In accordance with the present invention, this shortcoming maybe overcome. Of particular interest is the treatment of packaging orfood grade films such as those marketed under the name of SaranTM, andsimilar materials. For purposes of this discussion, the term"printability" refers to the acceptance of paint, dyes or similarmaterials, and hence includes dyability.

In accordance with the present invention, semiconductor wafers can betreated to etch the photoresist layer used in their manufacture. Forexample, a 4 inch diameter semiconductor wafer was etched with a gasmixture of 80% He and 20% O₂, for 5 minutes, at a discharge power of 100watts. The wafer was adhered to the uppermost electrode (see FIG. 6), toensure positive contact between the two structures. A negative pressureissuing from the face of the upper electrode was sufficient for thispurpose. An effectively etched wafer was obtained.

It is also contemplated that in accordance with the present invention,the objectionable static properties of various films and othersubstrates may be altered, allowing such materials to be handled easierand used for applications not previously permitted because of theirstatic properties.

When it is desired to treat elastomeric substrates to form elastomericfilms, sheets or webs, the substrates may include the polyurethanes,polyamids and polyesters disclosed, for example, in U.S. Pat. No.4,981,747, which is incorporated herein by reference. The formation ofelastic sheets from polyester elastic materials is disclosed, forexample, in U.S. Pat. No. 4,741,949, which is also incorporated hereinby reference. Likewise, elastomeric films or sheets may be made fromblock copolymers such as(polystyrene/poly(ethylene-butylene)/polystyrene) block polymers, as isalso disclosed in U.S. Pat. No. 4,981,747.

It will be noted that in accordance with the present invention, thesubstrate need not be exclusively made of synthetic material, but mayinclude non-synthetic material and may be in the form of laminates orcomposites including wood pulp, cellulosic materials such as cotton orrayon staple fibers, and other similar non-synthetic materialsfrequently used in composites or laminates.

It will therefore be understood that various changes in the details,materials and arrangement of parts which have been herein described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the following claims.

It should also be understood that various equivalent materials,structures or other means which perform substantially the same functionin a substantially manner to accomplish relatively the same result arewithin the scope of the invention.

What is claimed is:
 1. A method for generating a uniform, pulseddischarge plasma with a pair of electrodes, wherein each of theelectrodes has an electrode face and wherein the faces of the electrodesare in opposing spaced relation to each other, the method comprising thesteps of:(a) introducing a working gas into a working volume between theopposing faces of the electrodes; and (b) exciting the electrodes byapplying an asymmetrical primary voltage pulse with a pulse repetitionfrequency from about 1 to about 100 kHz to the pair of electrodes togenerate the uniform plasma in the working gas, wherein the asymmetricalprimary voltage pulse has an initial voltage rise followed by asubstantially faster voltage fall.
 2. The invention of claim 1, furthercomprising the step of treating a surface of a substrate with species ofthe generated plasma to alter one or more properties of the surface ofthe substrate.
 3. The invention of claim 2, wherein each of theelectrode faces has a plurality of holes, and which comprises the stepsof transporting the substrate between the pair of electrodes,discharging the working gas from the holes in one of the electrodefaces, and forming a vacuum through the holes in the other one of theelectrode faces.
 4. The invention of claim 2, wherein each of theelectrode faces has a plurality of holes, and which further comprisesthe steps of transporting the substrate between the pair of electrodes,and discharging the working gas from the holes in each of the electrodefaces.
 5. The invention of claim 2, wherein the substrate is treated fora period of time less than 60 seconds.
 6. The invention of claim 5,wherein the period of time is less than 20 seconds.
 7. The invention ofclaim 2, wherein the substrate is located outside of the working volumeand the plasma is convected out of the working volume and onto thesurface of the substrate.
 8. The method of claim 2 in which the surfaceof the substrate to be treated to alter one or more of its properties isa polymer selected from the group consisting of vinyl polymers,polyolefins, polystyrene, polyesters, nylons, silicones andpolycarbonates.
 9. The method of claim 8 in which the substrate of aspunbond polymer or a polymer film.
 10. The method of claim 2 in whichthe surface of the substrate to be treated to alter one or more of itsproperties is a natural material selected from the group consisting ofwool, cotton, leather and paper.
 11. The invention of claim 1, whereinthe working gas is maintained at or about one atmosphere of pressure.12. The invention of claim 1, wherein at least one of the electrodefaces has a plurality of holes which communicate with the working volumeand the working gas is passed through the holes in the electrode faceand introduced into the working volume at a determined temperature,humidity, and flow rate.
 13. The invention of claim 12, furthercomprising the step of causing turbulence in the working gas dischargedfrom the holes of the electrode face.
 14. The invention of claim 1,wherein the asymmetrical primary voltage pulse has an amplitude of atleast 1 kv, and a pulse width of 1 to 30 microseconds.
 15. The inventionof claim 1, wherein at least one of the electrodes has a gas receivingcavity separated into two portions by a partition, and which furthercomprises the steps of respectively introducing two different gases intothe two separate portions of the gas-receiving cavity, and developingtwo separate plasma discharges.
 16. The invention of claim 1, whereinthe plasma has a power density of at least 5 watts/cm³.
 17. Theinvention of claim 1, wherein the asymmetrical primary voltage pulse hasa rise time of approximately 1.25 microseconds.
 18. The invention ofclaim 1, further comprising the step of applying a magnetic field tointeract with the generated plasma.
 19. The method of claim 1 in whichthe plasma in the working gas has a power density of 50 watts/cm³. 20.The method of claim 1 in which the power density is in the range ofabout 18 to 26 watts/cm³.
 21. The invention of claim 1, wherein thevoltage fall is about 5 times faster than the initial voltage rise. 22.The invention of claim 21, wherein the initial voltage rise is about1.25 microseconds long and the voltage fall is about 250 nanoseconds.23. The invention of claim 1, wherein, following each asymmetricalprimary voltage pulse, is a secondary overshoot voltage pulse followedby one or more optional tertiary voltage pulses, wherein a dischargeplasma is generated during the asymmetrical primary voltage pulse, butnot during any tertiary voltage pulses.
 24. The invention of claim 23,wherein a discharge plasma is also generated during the secondaryovershoot voltage pulse.
 25. The invention of claim 1, wherein theasymmetrical primary voltage pulse is a negative voltage pulse.
 26. Anapparatus for generating a uniform, pulsed discharge plasma,comprising:(a) a pair of electrodes, wherein each of the electrodes hasan electrode lace, wherein the faces of the electrodes are in opposingspaced relation to each other to establish a working volume between theopposing faces of the electrodes; (b) a gas supply system configured tointroduce a working gas into the working volume; and (c) a pulsedischarge circuit configured to excite the electrodes to generate theuniform plasma in the working gas, wherein the pulse discharge circuitis configured to generate an asymmetrical primary voltage pulse with apulse repetition frequency from about 1 to about 100 kHz, wherein theasymmetrical primary voltage pulse has an initial voltage rise followedby a substantially faster voltage fall.
 27. The invention of claim 26,wherein at least one of the electrode faces has a plurality of holeswhich communicate with the working volume and the working gas is passedthrough the holes in the electrode face and introduced into the workingvolume at a determined temperature, humidity, and flow rate.
 28. Theinvention of claim 27, further comprising means for causing turbulencein the working gas discharged from the holes of the electrode face. 29.The invention of claim 26, further comprising means for transporting asubstrate between the pair of electrodes to adjust one or moreproperties of a surface of the substrate, wherein each of the electrodefaces has a plurality of holes, wherein the working gas is dischargedfrom the holes in one of the electrode faces, and wherein a vacuum isformed through the holes in the other of the electrode faces.
 30. Theinvention of claim 26, further comprising means for transporting thesubstrate between the pair of electrodes to adjust one or moreproperties of a surface of the substrate, wherein each of the electrodefaces has a plurality of holes, and wherein the working gas isdischarged from the holes in each of the electrode faces.
 31. Theinvention of claim 26, further comprising a coil adapted to control thegenerated plasma by regulating the temperature of the working gas,wherein the coil is associated with an electrode having an electrodeface with a plurality of holes.
 32. The invention of claim 26, whereinthe asymmetrical primary voltage pulse has an amplitude of at least 1kv, and a variable pulse width of 1 to 30 microseconds.
 33. Theinvention of claim 26, wherein at least one of the electrodes has a gasreceiving cavity separated into two portions by a partition, and whichfurther comprises means for respectively introducing two different gasesinto the two separate portions of the gas-receiving cavity, and fordeveloping two separate plasma discharges.
 34. The invention of claim26, wherein the pulse discharge circuit generates the plasma having apower density of at least 5 watts/cm³.
 35. The invention of claim 26,wherein the face of one of the pair of electrodes for generating theplasma is approximately hyperbolic in shape.
 36. The invention of claim26, wherein the asymmetrical primary voltage pulse has a rise time ofapproximately 1.25 microseconds.
 37. The invention of claim 26, furthercomprising a magnet adapted to apply a magnetic field to interact withthe generated plasma.
 38. The invention of claim 26, wherein the pulsedischarge circuit comprises a pulse generator connected to a pulsetransformer that converts pulse signals from the pulse generator intoconverted pulse signals that drive the electrodes during plasmageneration.
 39. The invention of claim 38, wherein the pulse dischargecircuit further comprises one or more diodes and one or more resistorsconfigured to control the shape of the converted pulse signals.
 40. Theinvention of claim 26, wherein the voltage fall is about 5 times fasterthan the initial voltage rise.
 41. The invention of claim 40, whereinthe initial voltage rise is about 1.25 microseconds long and the voltagefall is about 250 nanoseconds.
 42. The invention of claim 26, wherein,following each asymmetrical primary voltage pulse, is a secondaryovershoot voltage pulse followed by one or more optional tertiaryvoltage pulses, wherein a discharge plasma is generated during theasymmetrical primary voltage pulse, but not during any tertiary voltagepulses.
 43. The invention of claim 42, wherein a discharge plasma isalso generated during the secondary overshoot voltage pulse.
 44. Theinvention of claim 26, wherein the asymmetrical primary voltage pulse isa negative voltage pulse.